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Heraclitus was the philosopher who discovered the idea of change. Down to this time, the Greek philosophers, influenced by oriental ideas, had viewed the world as a huge edifice of which the material things were the building material. [...] As far as any processes were considered, they were thought of either as going on within the edifice, or else as constructing or maintaining it, disturbing and restoring the stability or balance of a structure which was considered to be fundamentally static. They were cyclic processes. [...] Heraclitus introduced the view [...] of the world [...] as one colossal process, [...] as the totality of all events, or changes, or facts. [...]
If all things are in continuous flux, then it is impossible to say anything definite about them. We can have no real knowledge of them, but, at the best, vague and delusive 'opinions'. [...]

Parmenides [...] had taught that the pure knowledge of reason, as opposed to the delusive opinion of experience, could have as its object only a world which did not change, and that the pure knowledge of reason did in fact reveal such a world. But the unchanging and undivided reality which Parmenides thought he had discovered behind the world of perishable things was entirely unrelated to this world in which we live and die. It was therefore incapable of explaining it. [...]

Socrates was interested in ethical matters; he was an ethical reformer, a moralist, who pestered all kinds of people, forcing them to think, to explain, and to account for the principles of their actions. He used to question them and was not easily satisfied by their answers. [...] He was led to enquire into the 'virtue' of a thing. 'It was natural', says Aristotle, 'that Socrates should search for the essence', i.e. for the virtue or rationale of a thing and for the real, the unchanging or essential meanings of the terms. 'In this connection he became the first to raise the problem of universal definitions.' [...]

Plato developed Socrates' method of searching for the meaning or essence into a method of determining the real nature, the Form or Idea of a thing. [...] Though there 'could be no definition of any sensible thing, as they were always changing', there could be definitions and true knowledge of things of a different kind -- of the virtues of the sensible things. 'If knowledge or thought were to have an object, there would have to be some different, some unchanging entities, apart from those which are sensible', says Aristotle, and he reports of Plato that 'things of this other sort then, he called Forms or Ideas, and the sensible things, he said, were distinct from them, and all called after them. And the many things which have the same name as a certain Form or Idea exist by participating in it.'
Plato's fundamental problem was to find a scientific method of dealing with sensible things. He wanted to obtain purely rational knowledge, and not merely opinion; and since pure knowledge of sensible things could not be obtained, he insisted on obtaining at least such pure knowledge as was in some way related, and applicable, to sensible things. [...] The Form was the accountable representative of the sensible things, and could therefore be consulted in important questions concerning the world of flux.

(K.R. Popper: The Open Society and its Enemies (1962), I, Ch. 2 and 3)